Taller tires - computer recalibration

Gm only gives the dealer the ability to program the truck with the possible tire combinations available for that truck... meaning is it possible to get a truck with that engine and chassis with that size tire?? if not , then gm cannot program the vehicle... you also have to program the abs and pcm separately... some dealers just program the abs, which is much more lenient, and the shift points and Speedo are still wrong...If you find someone who deals in tires, you may be able to find a match of circumference of tires between what you have and what is available.... as for aftermarket, I don't know how they do it... must kinda be like hacked firmware for a dvd player or something....probably works, I just have no experience with it... Bobo

P.S. If you plan on leaving it as it is... I would try to have the dealer program the abs for the brakes.. the small change in speed may have a dramatic effect in how the abs works... I know it's a small speed change, but with thousands of pounds here it wouldn't take a lot to throw a monket wrench in the works....

difference,

Reply to
Bobo
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Approximately 11/25/03 14:07, Jason Cothran uttered for posterity:

Whats the increase in circumference? Ignoring the fact that unless you know the brand and model of tire, you really don't have anything more than a general idea of the actual tire diamters involved...

Reply to
Lon Stowell

Fun, yes. Especially when your formula calculates area.

Circumference = PI times diameter

Complicating things a bit more, the rolling circumference of a tire is not the same thing as one would calculate based on "tire size." The rolling radius is shorter because the tire deflects under load. However what you suggest is a good approximation especially when looking to approximate the differences between two different tire sizes.

Best thing to do is to consult the tire manufacturer's specs. Should be able to get a very accurate rolling circumference. Is usually pre-calculated and stated in revolutions per mile.

Reply to
David Kelly

OOPS!!

My bad. still you get the idea.

Reply to
rnf2

Leaving it as is will affect your 'perceived' gas mileage and distances traveled along with a speedometer correction.

As I mentioned, I went one size over anything that was a dealer option for my Jeep and when I checked the speedo out with a GPS unit, it turned out to be right on exactly.

They usually calibrate the speedo from the factory to be slightly over with the largest 'factory option' tires so when the speedo says 65 mph, you are 'really' only going 63 or 64 mph.

I would just ask the dealer to dial the speedo for the biggest tire they can and see what the result was.

Meanwhile, all us Jeep folks with really large tires usually just get paced to find out where the speedo should be and judge the speed accordingly. It wouldn't hurt my feelings to know I have to be at 63 mph on the speedo to be doing the legal 65 mph. My CJ7 was way off when I got it with 31" tires on, it read 65 and I was only going 60 before I went up to 33x9.5 tires. So I just drove at '70 mph' to keep up the speed limit.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

mxz1972 wrote:

Reply to
Mike Romain

Yo bud, don't know what the heck you are measuring, but in the BFG line, a P215/75/R15 has a diameter of 27.8".

The P235/75/R15 has a diameter of 29.0"

Just a wee bit more than a 1/2" eh....

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Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Reply to
Mike Romain

Think again. that is .6inches (just a wee bit bit over 1/2 inch) per side. It is simple math. The aspect ratio is simply a percentage of the total width. E.G., a 215/75 is 215mm*.75 inches tall (per side =*2, add rim diameter). a 235/75/15 is 235mm*.75 inches tall (per side =*2, add rimdiameter), so yes a 235/75 is, like I said, just a wee bit over a 1/2 inch (per side, but since we are all assuming, I thought you knew that) taller than a 215/75. You even showed it yourself off the chart (29-27.8=1.2 / 2 = .6 inches per side (radius)).

Reply to
Jason Cothran

Approximately 11/26/03 14:38, Jason Cothran uttered for posterity:

Ah, but what is the circumference difference little grasshopper?

87.3 compared to 91.1 inches, or a 3.8 inch difference in circumference, or just a tad over 4%, not enough to get you a speeding ticket unless you urinate on the ossifer's foot just before you beer barf on his nicely pressed uniform, but still enough to bring a slightly out of true speedometer into line.

Only for a pure and hypothetical tire, little grasshopper. Real tires are spec'd to the closest standard value and grasshoppers are encouraged to consult the actual manufacturer's data for the real tire dimensions. And recall that the difference in diameter is multipled by 3 in Kentucky and 3.14 elsewhere.

Reply to
Lon Stowell

Ah yes, the make it any size you want to rule lol . Where an inch equals a mile and pi is forgetten :-D.

Reply to
Jason Cothran

Huh???

I really don't get what you are saying....

The damn tire is over 1" taller. Period. Put the two against a wall and compare, one 'will' be 1.2" taller than the other on the same rim.

In my book that makes it a 'lot taller'. A 'lot taller' enough that it won't fit on some vehicles too.

The radius has no bearing on this, nor is it what I said in the first place.

Now if I wanted to figure out how much more ground clearance I would get, I would divide the diameter in half. In my case on my CJ7, I got

1" more ground clearance by changing from 31's to 33's. I got 2" more diameter which is the number needed to figure a speedo.

On my Cherokee I only got .6" more ground clearance changing from P215's to P235's.

If I am figuring out the speedo, I want the diameter measurement to get the circumference difference.

If you can't understand this simple math, then I give up on you.

I have no urge to argue with someone that wants to talk in circles.

Bye now....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
Reply to
Mike Romain

determines how

rimdiameter), so yes

Reply to
Steve W.

the radius has EVERYTHING to do with it 'being taller'

the RADIUS is half of the DIAMETER

Reply to
Gary Glaenzer

ROTFLMAO!!!!

Man, so why would I want to divide the diameter in half to figure out the circumference?

I don't want the area of the sucker, I want the circumference to figure the % difference from the smaller tire to the larger.

The speedo will be off by that percent....

Mike

Gary Glaenzer wrote:

Reply to
Mike Romain

It isn't 'my' tires in question here, someone else's, I didn't start this thread.

And as it turns out, I am now running 'exact' to my GPS with the oversized tires on it.

The factory speedo gear was giving me a 'safe' speedo reading, which was low compared to 'reality'.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

"Steve W." wrote:

Reply to
Mike Romain

did you or did you not say that 'the radius has no bearing on this?'

and you don't need to know the circumference..............where S = smaller tire and L = larger:

circumference L / circumference S = diameter L / diameter S = radius L / radius S

they are in EXACTLY the same ratios since circumference = 3.1416 diameter =

6.2832 radius

Reply to
Gary Glaenzer

Because the tire is 2 * PI radians around. And the circumference is

2 * PI * R.

Punch some buttons on your calculator. For percent changed the radius works exactly as well as the diameter which works exactly as well as the circumference when comparing speedometer/odometer errors.

A high school student in algebra class should be able to explain why. Because we are lacking HS algebra graduates I'll explain.

The only difference between circumference and diameter is that diameter is multiplied by PI to get circumference. When comparing two tires the constant PI can be factored out from both sides of the equation.

Same applies for radius and 2 * PI.

Reply to
David Kelly

Ouch, ok.... I give up. LOL!

I like simple stuff......

You know, the 'KISS' principal.

If I put a BFG P235/75 against a wall and a BFG P215/75 against a wall I have a 1.2" height difference.

In my mind that makes one tire taller than the other. 'Lots taller'.

The chart at BFG tells me this.

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So I have a 'diameter' given to me from the tire maker, no radius mentioned there....

WHY THE HECK SHOULD I DO ANY FANCY MATH CONVERSIONS WHEN THE FORMULA IS SO SIMPLE! (or even need to know more than basic, which I happen to anyway)

Excuse me for shouting, but this thread is getting totally silly....

PI times the diameter works for me......

Radius does not come into the equation when figuring out a speedometer like the original poster asked about!!!!

The tire makers don't give you radius on the tire charts, they give diameter!

Crap guys....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

David Kelly wrote:

Reply to
Mike Romain

Mike, determining tire height with radius suits better for those who want more academic conversation and more "convincing" formula. They have the joy of multiplying the radius by two to get what we know without multiplying, "how tall the tire is", diameter.

Reply to
Jukka

Reply to
George

You're the one that admitted you dont get. rofl.

Reply to
Jason Cothran

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